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If Tropical Cyclone Oma collides with New Zealand the destruction will be "devastating" and similar to that of Cyclones Fehi and Gita, MetService says.
However, predictions are still unclear as to where the slow-moving system is tracking, and it will be another two days until forecasters can say with some certainty what will happen.
Meteorologist William Nepe said the category two system was near New Caledonia at the moment, where winds were blowing at 92km/h.
"There is still a great deal of uncertainty as to where it will track, it is at the moment slowing in the south-west, moving at about 6km an hour.
"It could either go west towards Australia or it could come east towards New Zealand.
"Some solutions have it moving towards New Zealand and recurving towards the north," Nepe said.
"One thing we know with confidence is it will move slowly for the next few days."
The power of the cyclone, if it hit New Zealand, would be similar to that of Cyclone Gita and Fehi which pounded the country last year.
During Gita, Christchurch, Buller District, Grey District, Selwyn, Westland, Tasman and Taranaki declared a state of emergency as the country was bashed by 15m waves, more than 300mm of rain, and wind gusts above 130km/h.
The cyclone should continue to move south-southwest during Wednesday and Thursday, to lie over the southern Coral Sea by the end of Thursday.
"I think it won't be until Thursday or Friday to know definitely," Nepe said.
The system, as well as heavy rain and flooding, would come over the weekend at the same time there was potential for king tides.
"That's if it were to become a deep low, it depends on what system we have interacting with it."
"The worst-case scenarios are quite devastating, some of the model winds had 80-knot winds (158km/h) near the surface and 110 knots (203km/h) at several thousand feet.
"It is quite alarming in the worst case scenario."
Weatherwatch also confirmed the uncertainty of the tropical cyclone's path but could confirm that a new low forming around New Zealand would likely bring bouts of heavy rain.
"This means a large new low with some connections to the tropics and cold air from the Southern Ocean will help produce a sizeable low-pressure system bringing rain to both islands. But where, how much and when still cannot be locked in due to so much chaos.
"This is quite a complicated set-up."
The world "two most trusted sources" for cyclones were still at odds with the storm's path after Friday.
"GFS [Global Forecast System] still picks the tropical low directly affecting New Zealand while ECMWF [European Centre for Medium-Range Weather Forecasts] shows Oma doing a complete U turn and reversing back into the tropics this weekend."